1. Field of the Invention
The present invention relates to a yaw moment control system in a vehicle, in which the steering characteristic is changed by distributing different amounts of torque to left and right wheels.
2. Description of Related Art
There is a conventionally known yaw moment control system in a vehicle which has already been proposed by the present assignee and which is designed to control the yaw moment by generating a driving force in one of the left and right wheels of the vehicle which are connected to each other by a gear box and a torque transmitting clutch, and by generating a braking force in the other wheel. An undesirable yaw moment generated upon acceleration or deceleration of the vehicle which is being turned, is eliminated by setting the distribution amounts of the driving force and the braking force as a function of a product of the longitudinal acceleration and the lateral acceleration (see Japanese Patent Application Laid-open No. 9-86203).
As known by the theory of a tire friction circle, a grip force applied to the grounded face of a tire is composed of a longitudinal driving force (a braking force) and a lateral cornering force. The resultant force cannot exceed a static friction force on the grounded face.
Therefore, if a driving force is applied to the front wheels which are the driven wheels to increase the longitudinal acceleration during the turning of a front wheel drive vehicle at or near the limit of the grip force of the tire, the cornering forces of the front wheels are decreased in accordance with the increase in the longitudinal acceleration. The vehicle which is being turned is maintained in stability about a yaw axis by the balance between the cornering force of the front wheels and the cornering force of the rear wheels. For this reason, there is a problem that a front portion of the vehicle is pushed outwards in the turning direction due to the above-described decrease in the cornering force of the front wheels to thereby intensify an under-steering tendency. Especially, if the grip force of the tire has reached its limit, thereby causing the slip rate to be increased, the driving force is slowly decreased, whereas the cornering force is rapidly decreased, and for this reason, the under-steering tendency rises significantly.
In contrast, if the driving force is applied to the rear wheels which are driven wheels, to increase the longitudinal acceleration during turning of a rear wheel drive vehicle at or near the limit of the grip force of a tire, the cornering force of the rear wheels is decreased in accordance with the increase in the longitudinal acceleration. The vehicle which is being turned is maintained in stability about the yaw axis by the balance between the cornering force of the front wheels and the cornering force of the rear wheels. For this reason, there is a problem that a rear portion of the vehicle is pushed outwards in the turning direction due to the above-described decrease in the cornering force of the rear wheels to thereby intensify an over-steering tendency. Especially, if the grip force of the tire has reached its limit, thereby causing the slip rate to be increased, the driving force is slowly decreased, whereas the cornering force is rapidly decreased, and for this reason, the over-steering tendency rises substantially.